1. Field of the Invention
The subject matter of the present application relates to semiconductor processing and more specifically to a method for providing improved alignment for making a photolithographic exposure.
2. Description of the Related Art
Photolithography refers to a process of transferring geometric shapes on a photomask or reticle (“mask”) to a semiconductor wafer or layer on the wafer. Typically, a photolithography exposure tool is used to cast an image of the mask shapes onto a photoresist layer on the wafer, and then, once the imaged photoresist layer is developed, the resulting photoresist patterns can be transferred to another layer of the wafer, such as by etching. During processing, a wafer can undergo a photolithography step to form patterns in one layer of the wafer, and then undergo a subsequent photolithography step to form patterns in another layer of the wafer above the earlier patterns. Sometimes, both the first and subsequent photolithography steps can be performed using the same photolithography exposure tool. More commonly, however, different photolithography exposure tools are used. The use of different exposure tools presents challenges because each tool can cause distortion and misalignment of the exposure image with the patterns formed on the wafer by an earlier used tool. Moreover, as each exposure tool can have more than one chuck to hold the wafer in the exposure tool, alignment error can occur in a way that is unique to each chuck.